1. Field of the Invention
This invention relates to a method for manufacturing a polyimide sleeve which can be used for a fixing apparatus of an electrophotograph copying machine and the like.
2. Description of the Related Art
In a copying or printing unit such as an electrophotograph copying machine, a facsimile or a laser beam printer, a heat-fixing system is generally adopted where a toner image of a hot-melt type resin etc. is formed on a recording sheet by an image forming process such as an electrophotography, an electrostatic recording, a magnetic recording, or the like so as to be fixed by heat.
For a fixing apparatus used in these heat fixing systems, a heat roller system has been conventionally utilized where a recording sheet on which a toner image is formed, is fed between two rollers of a heater-imbedded fixing roller and a pressure roller so as to fix the image, while a thin film belt fixing system has been developed and widely utilized recently where a film-shaped seamless sleeve (a polyimide sleeve) made of polyimide, polyamide-imide, or the like, or a sleeve whose outside surface is covered with a fluoropolymer is used in stead of the fixing roller.
In the fixing apparatus according to this system, a heater is placed inside of the polyimide sleeve, and the polyimide sleeve is pressed to the heater and a pressure roller is pressed to an outside of a portion of the polyimide sleeve which is pressed to the heater. Then a recording sheet, on which a toner image is formed, is passed through between portions of the polyimide sleeve which is pressed to the heater and the pressure roller, and the recording sheet is heated through the polyimide sleeve so that a toner image is heat-fixed.
Since a heat capacity is small for the polyimide sleeve which is a thin film, the system is characterized in that heat fixing can be available right after the power turned on as well as a required energy for heating can be decreased.
Additionally, a fixing apparatus using the above-described polyimide sleeve instead of the pressure roller has been developed recently.
In this fixing apparatus, a polyimide sleeve suspended among a plurality of rollers is pressed to an outer surface of a heater-imbedded fixing roller, and while rotating the fixing roller and the polyimide sleeve, a recording sheet on which a toner image is formed is passed through therebetween so as to be heat-fused.
Since an adequate fixing time can be secured even when transfer speed of a recording sheet is increased, this method is characterized in that speeding up of a machine can be achieved.
As for a method for manufacturing a seamless polyimide sleeve in use for these fixing systems, such a method is typically known that a polyimide precursor solution such as a polyamide acid solution is applied on an outside surface of a cylindrical metal core, and after being dried and subjected to an imide reaction by heating, a sleeve-like member is released from its core.
However, since polyimide is a polymer used as an adhesive, when the polyimide precursor solution is applied and subjected to the imide reaction by heating, the polyimide sleeve shrinks to fit strongly or adhere to the core, and it becomes difficult for the polyimide sleeve to be removed from the core.
Therefore, in order to make such a release from the core easy, methods have been further proposed as follows; (1) a method of applying a polyimide precursor solution on a core, heating it till sufficient strength at least as a sleeve-like member is obtained, and after released from the core once, baking it while being inserted again onto the core, (2) a method of providing small holes on a core, applying a polyimide precursor solution on the core, and after heated and baked, supplying an air pressure through the small holes from the inside of the core so that the sleeve is released from the core, (3) a method in which a release material such as a silicone oil is applied on a surface of a core, or the like.
However, in the method of (1) where a step of heating operation is repeated twice, there exist difficulties that the process is so complicated that productivity will decrease and that a yield of a product will also decrease since the sleeve is released and re-inserted during the imide reaction.
In the method of (2) where small holes are provided on the core, there exist difficulties that manufacturing cost of the core will increase while quality and a yield thereof will decrease since film is easy to come off and bend because of the air pressure given against a shrinking force of the sleeve.
In the method of (3) where the release material is applied on the surface of the core, such a new problem arises that while making it easy to separate the sleeve from the core, the core surface itself becomes hard to get wet, so that a crawling phenomenon and a flow phenomenon occur when a polyimide precursor solution is applied, whereby it will be difficult to apply it evenly in a desired thickness.
For this problem, methods have been proposed for making it easy to separate the sleeve from the core and repressing an occurrence of a crawling phenomenon, by covering a surface of the core with a heat-resistant resin of small interfacial contact angle in Japanese Patent Laid-Open Application No. 3-261518 JP-A for example, and by providing an inorganic coating layer on the surface of the core in Japanese Patent Laid-open Application No. 7-76025 JP-A. However, in both cases there exists a difficulty that making defects or coming off is likely to be caused since the surface strength of the covering layer and the adhesive strength to the core are not enough. With a few defects, a pinhole or a scar would be caused on the polyimide sleeve, which results in quality loss.
As noted above, while various modifications have been conventionally attempted to release the polyimide sleeve from the core easily, they have respective difficulties and a technique has not been established yet to easily manufacture a high quality polyimide sleeve with a superior yield.